The invention relates generally to a holographic method for correcting anomalies in a coherent light beam containing rays of a first wavelength and more particularly to a holographic method for correcting anomalies in an infrared (IR) beam from a laser diode or a laser diode array.
In the past, consideration has been given to the use of holograms in lieu of traditional optical elements for correcting aberrated wavefronts from laser diodes or laser diode arrays to produce well-collimated beams suitable for use in free space optical communications between satellites.
Laser diodes or laser diode arrays are uniquely suited to spacecraft communications because of their small size and mass. However, they do present problems. Single diode lasers produce diverging cones of light that are highly astigmatic. These devices also lack the power needed for a communications system. Coherent light producing diode arrays have sufficient power, but produce rapidly diverging cones of light with highly irregular wavefronts. Thus, the radiation from such a device must be transformed by an appropriate optical system which corrects the aberrated wavefront, collimates the beam, and directs the beam toward the receiver. Conventional optical systems are presently used to transform the beams radiated by single diode lasers. Such systems include many lenses and an anamorphic prism pair to correct the astigmatism. These elements of the system are quite massive for satellite use and the lasers produce light inadequate in power for various potential applications. On the other hand, no really suitable optical system is available to efficiently transform beams from laser diode arrays because of the complexity of the beam's wavefront. Thus, the higher power available from these arrays is not fully available for use.
It would be a great advantage if holograms could be used in place of conventional optics to transform these laser beams. Holograms are thin, low in mass, and rugged. A single lightweight hologram can perform the same operations as a heavier complex optical system. Furthermore, single hologram can perform operations which cannot be preformed by conventional optical systems. It appears possible that holograms can be used to efficiently correct complicated wavefront aberrations in a beam from a laser diode array. Existing literature addresses the feasibility of the application of holograms as beam transformers in satellite communications links together with some difficulties to be overcome and possible solutions have been mentioned.
The straightforward approach to obtaining a suitable hologram would be to expose a holographic emulsion to the interference pattern formed between the aberrated light cone radiated by a laser diode or a laser diode array and a replica of a good communications beam obtained by beam splitting, optically correcting, and collimating light from the same source. The hologram would serve in lieu of a traditional optical system to transform the light cone into the communications beam. However, the difficulty with this approach is the unavailability of high-resolution emulsions capable of producing efficient holograms at the wavelength of interest (e.g., an infrared wavelength), which are sensitive at the wavelength of interest. Moreover, achieving a diffraction-limited optical beam with a hologram is not straightforward. Two critical design considerations in the construction of holographic optical elements are efficiency, .eta., and sensitivity. Hologram efficiency (.eta.) involves the ratio of usable light to the total light illuminating the holographic medium. If a hologram is illuminated with 10 mW but only 1 mW is available for use after striking the hologram the efficiency (.eta.) is just 10%. For spacecraft communications, where total output power is a critical parameter, high efficiency (.eta.) is required.
Wavefront fidelity is best achieved when a hologram is illuminated with one of the beams used to create it. Consequently it is important to record the actual wavefront in the medium itself. However, presently holographic emulsions which diffract efficiently are not sensitive in the infrared (IR). These materials are typically sensitive in the blue. New polymers are sensitive in the red. Consequently, direct recording of the actual wavefront required for the best recovery of the desired beam is not possible with the straightforward approach.